1. Field of the Invention
The present invention relates to a method of processing an optical device wafer by which an optical device wafer having optical devices composed of a gallium nitride compound semiconductor or the like layered in a plurality of regions demarcated by streets formed in a grid pattern on the face-side surface of a substrate such as a sapphire substrate is divided along the streets into the individual optical devices.
2. Description of the Related Art
An optical device wafer in which a plurality of regions are demarcated by planned dividing lines called streets formed in a grid pattern on the face-side surface of a sapphire substrate and optical devices composed of a gallium nitride compound semiconductor or the like are layered in the demarcated regions is divided along the streets into the individual optical devices such as light emitting diodes, which are widely utilized for electric apparatuses. As a method for dividing the optical device wafer along the streets, a method of cutting the optical device wafer by a cutting blade rotated at high speed has  been attempted, but it is difficult to cut the sapphire substrate by a cutter, since the sapphire substrate has a high Mohs hardness and is a difficult-to-cut material.
In recent years, as a method for dividing a wafer such as an optical device wafer along streets, there has been proposed a method in which the wafer is irradiated with a pulsed laser beam having such a wavelength as to be absorbed in the wafer along the streets to form laser beam-machined grooves, and an external force is applied along the laser beam-machined grooves so as to break up the wafer along the streets (see, for example, Japanese Patent Laid-open No. Hei 10-305420).
In addition, as a method for dividing a wafer such as an optical device wafer along streets, there has also been proposed a method in which the wafer is irradiated with a pulsed laser beam having such a wavelength as to be transmitted through the wafer along the streets while adjusting the focal point position of the pulsed laser beam to the inside of the wafer, thereby continuously forming an altered layer inside the wafer along the streets, and an external force is exerted along the streets along which the wafer strength has been lowered due to the formation of the altered layer, to break up the wafer along the streets (see, for example, Japanese Patent Laid-open No. 2008-6492).
The optical device wafer as above-mentioned is formed to a predetermined thickness by grinding the back side thereof, prior to division into the individual devices. Besides, in recent years, a reduction of the thickness of optical devices to 50 μm or below is demanded for realizing electric apparatuses reduced in weight and size. When an optical device wafer is ground to a small thickness of 50 μm or below, however, the optical device wafer would be cracked.